When no-load energizing is performed by supplying power in a state in which there is residual magnetic flux in the transformer core, a large magnetizing inrush current flows. It is widely known that the magnitude of this magnetizing inrush current can be several times the rated load current of the transformer.
When such a large magnetizing inrush current flows, the system voltage fluctuates, and when this voltage fluctuation is large, consumers can be affected.
As a method of the prior art for suppressing magnetizing inrush currents, a magnetizing inrush current suppression method is known in which a circuit breaker with a resistor, formed by connecting in series an closing resistor and a contact, is connected in parallel with any of the circuit breaker main points, and power is turned on to this circuit breaker with a resistor in advance of the circuit breaker main point (see for example Patent Document 1).
As another method of suppression, a method is known in which, when a three-phase transformer of a directly grounded system is supplied with power using three single-phase circuit breakers, one arbitrary phase is supplied with power in advance, and thereafter the remaining two phases are supplied with power, to suppress magnetizing inrush currents (see for example Non-patent Document 1).
Patent Document 1: Japanese Patent Application Laid-open No. 2002-75145, “Gas Circuit Breaker with Device for Suppression of Magnetizing Inrush Currents”
Non-patent Document 1: IEEE Trans., Vol. 16, No. 2, 2001, “Elimination of Transformer Inrush Currents by Controlled Switching—Part I: Theoretical Considerations”
In the case of the magnetizing inrush current suppression method disclosed in the above-described Patent Document 1 employing a circuit breaker with a resistor, because it is necessary to specially add a circuit breaker with a resistor to the ordinary circuit breaker, in terms of the circuit breaker as a whole, larger equipment size is undeniable.
Moreover, as is well known, there exist non-phase segregated operation-type circuit breakers designed to simultaneously perform circuit breaker open/close operations of three phases using a single operation mechanism; but such non-phase segregated operation-type circuit breakers have the drawback that the magnetizing inrush current suppression method disclosed in Non-patent Document 1 cannot be applied.
On the other hand, as described in the above Non-patent Document 1, it is important that, when suppressing magnetizing inrush currents at the time of supplying power of transformer, the magnitude of the residual magnetic flux when the transformer is shut off be ascertained.
When a circuit breaker shuts off at the zero point the magnetizing current flowing in a no-load transformer installed in a non-effectively grounded system, after shutting off the first phase a zero-phase voltage appears, and after shutting of the second and third phases the zero-phase voltage becomes a DC voltage and remains on the transformer. Consequently when the voltage to ground at each of the transformer terminals on the side shut off by the circuit breaker is being measured using a voltage measuring device, the above-described DC voltage is measured after shutoff.
The residual magnetic flux in the transformer core can be determined by integration of the voltage. For example, in the case of a Y connection, if the voltages across each terminal and a neutral point are measured and integrated, the residual magnetic flux in the transformer core can be calculated accurately, without being affected by the above-described DC voltage.
However, in general voltage transformers (VTs, PTs) and capacitor type voltage transformers (PDs), and other voltage measuring devices which voltage-divide high voltages for voltage measurement, are connected across the transformer terminals and ground. Such voltage measuring devices are able to measure the voltage to ground at the transformer terminals, and upon integrating these voltages, the integration result includes the above-described DC voltage, so that the integration value diverges, and the residual magnetic flux cannot be accurately determined.